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Other details included the rank of the officer leading the investigation, whether he or she worked in a specialised police area and the amount of time spent carrying out the inquiry.

Detective Inspector David Kirby, manager of the National Criminal Investigations Group, said: "The investigation is still ongoing and whilst the investigation is ongoing police is not in a position to go into specific details of the complaint."

Kirby quoted the section of the Act relied on to refuse providing the information, which says OIA requests can be knocked back if doing otherwise would "prejudice the maintenance of the law, including the prevention, investigation, and detection of offences, and the right to a fair trial".

Other areas police ruled out were the date on which Ross had been told there was an investigation, whether he had been interviewed - if at all - and whether Cabinet ministers had been told of the inquiry.

The blanket withholding of basic information, commonly released by police, was at odds with normal practice, said a former detective, who would not be named.

He said details such as part of the country where the investigation was based, or rank of the officer, would not cause problems to the investigation or any prosecution to which it led.

Instead, withholding such details gave the appearance of the investigation being treated differently from the way others would be handled.

Ross, 33, revealed the investigation just before his return to Parliament this year. It was a move which led to Dowie being named as one of the women with whom he had an extra-marital relationship while National MP for Botany.

Ross was kicked out of the party during a tumultuous week in October during which he accused National Party leader Simon Bridges of inappropriate handling of a donation. He followed the claim with a formal complaint to police which is ongoing.

During that week Ross was accused of adverse behaviour towards women, and his mental heath deteriorated.

It was during this period Ross said he reflected on a text message received from Dowie's phone two months earlier which led him to consider self-harm.

Neither Dowie or Ross responded to calls for comment.

Supporters on the campaign trail for Sarah Dowie MP. Photo / Supplied

Her electorate neighbours that once held by former MP Todd Barclay, who faced police investigation into allegations he had secretly recorded a staff member.

Barclay refused to be interviewed by police. The investigation closed with no charges laid while Barclay chose not to stand for Clutha-Southland again in 2017.

WHERE TO GET HELP:

If you are worried about your or someone else's mental health, the best place to get help is your GP or local mental health provider. However, if you or someone else is in danger or endangering others, call police immediately on 111.